24 



MARINE AND FISHERIES 



1-2 EDWARD VII., A. 1902 



forward. The lower limb or brown, broad, thick-walled glandular portion bends 

 upwards at its anterior end opening into the pericardial cavity, while the lower limb 

 or thin-walled, non-glandular part bends downwards at its anterior end crossing the 

 other portion and opening into the cloacal chamber. Lying in the mantle and body 

 walls, near the anterior end of the pericardium, is the pericardial gland, red-brown 

 organ or organ of Keber. It is thought to be also excretory in function. 



Circulatory System. — The heart is situated in the pericardial cavity. It is 

 composed of a median, thick-walled ventricle, pierced by the rectum, and a thin-walled 

 auricle on each side, opening into the ventricle. Anteriorly and posteriorly the ventricle 

 gives origin to aortse, which divide into smaller arteries, distributing the blood to the 

 mantle and the body. The mantle acts as a respiratory organ upon the blood, which 

 is collected and conducted through vessels directly to the auricles ; but the blood that 

 goes to the capillaries of the different organs of the body is collected into a large vein 

 lying between the nephridia, from which it must first pass through a capillary net- work 

 in the walls of the kidney and then through the capillaries of the gills before it is 

 carried as arterial blood to the auricles, whence it passes with that from the mantle into 

 the ventricle. 



Nervous System. — Cerebral ganglia connected by a commissure, lie one on each 

 side of the oesophagus. Each of these is united by connectives with the pedal ganglion 

 situated in the base of the foot, and with the visceral ganglion situated in front of the 

 posterior adductor muscle. Both pedal and visceral ganglia show indications of being 

 double, like the cerebral ganglia. From each cerebral ganglion spring two nerves — a 

 short one supplying the anterior muscles, and a long one running forwards and down- 

 wards to the border of the mantle, where it divides into inner and outer parallel nerves. 

 These course round the mantle rim and unite before entering the visceral ganglion. The 

 outer one gives off twigs behind to the siphons. From the visceral ganglion arise nerves 

 to the posterior muscles and to the gills. (Plate IV., Figs. 5, 6.) 



It will be observed that the clam is bilaterally symmetrical, in that a vertical 

 cleavage, falling along the median longitudinal axis, would divide the animal into similar 

 right and left halves. The shells, the mantle lobes, the gills, palps, auricles, nephridia, 

 genital openings and cerebral ganglia are paired, right and left ; while those organs 

 which lie in the median plane of the body, such as the foot, intestine, ventricle, are 

 unpaired or single. As in a great many other mollusks, however, the valves of the shell 

 present more or less of an asymmetry in consequence of their bilaterality not being 

 absolute. 



NEAREST RELATIVES OF THE CLAM. 



* Clams or clamps is a shellfish not much unlike a cockle ; it lieth under the sand. ' Wood, 1684. 

 The term ' clam ' is applied to at least a dozen different species of American double- , 

 shelled animals. To distinguish these, qualifying expressions are frequently used. 

 Most of the names of the species Mya arenaria (Linnaeus, 1758) here dealt with are the 

 following : — 



The clam. 

 The common clam. 

 The long clam. 

 The soft clam. 

 The soft-shelled clam. 

 The sand clam. 

 The squirt clam. 

 The maninose clam. 

 The nanninose. 



In England it is called : — 



Gaper clam. 

 Sand gaper. 

 Old maid, &c. 



